The present invention relates in general to drive systems for electric vehicles, and, more specifically, to improved precharging of a main DC bus link capacitor.
Electric vehicles, such as hybrid electric vehicles (HEVs) and plug-in hybrid electric vehicles (PHEVs), use inverter-driven electric machines to provide traction torque and regenerative braking torque. Such inverters typically employ an energy storage capacitor (or the main capacitor) as the DC link for a main DC bus, which is usually interfaced with the high-voltage (HV) power source such as a battery through a variable voltage converter (VVC), an input capacitor, and a pair of mechanical contactors (e.g., relays). An inverter and other loads are driven from the main DC bus.
If the contactors are initially closed with the link capacitor in a discharged or low charged state, a low impedance from the HV DC source to the main DC bus can result in a very high inrush current that could cause damage to the contactors and other components. Use of a current-limiting resistor in series with the contactors is undesirable because of the associated voltage drop and power consumption during subsequent normal operation. Therefore, a separate circuit branch, or precharging circuit, is often used. The known precharging circuits utilize a switch and a resistor in series between the DC supply and the link capacitor. Turning on the switch allows the link capacitor to be charged through the resistor, and the presence of the resistor limits the inrush current to prevent damage to the switch. Once the link capacitor is precharged, then i) the main contactors can be closed without receiving any inrush current and ii) the precharge switch can be opened so that the precharge resistor is disconnected.
It is desirable to complete the precharging process in a short amount of time so that the vehicle can be driven immediately after the driver activates it. The charging time of the capacitor in the conventional arrangement is governed by the RC time constant of the precharging circuit and link capacitor. Since the precharging resistor must be large enough to limit inrush current and the link capacitor necessarily has a relatively large capacitance, an undesirably long delay has sometimes occurred. Furthermore, the presence of additional loads on the main DC bus can affect the precharging by increasing the impedance. For example, a bleeder resistor is typically present across the link capacitor to discharge the link capacitor during shutdown of the electric drive. Other possible loads include an electric (PTC) heater. The loads may further prolong the precharging time.
The effective resistances of the loads may change over time, and some loads such as an electric heater could be switched on or off when performing a precharge. The capacitance of the link capacitor may also degrade over time. These variations have made it more difficult to ensure that a precharge is completed within a predictable amount of time.
For diagnostic and monitoring purposes, it is desirable to measure the capacitance and resistance associated with the main DC bus throughout the lifetime of the electric drive. Dedicated components have typically been required in order to perform these measurement functions. It would be desirable to perform such measurements without requiring dedicated components.